An efficient process exists to ensure that only virion RNA is incorporated into tobacco mosaic virus (TMV) particles; assembly is initiated by reaction of a capsid protein oligomer with a specific viral RNA encapsidation initiation (ei) site. Nevertheless, TMV preparations contain a small proportion of pseudovirions, particles that contain host rather than viral RNA. Pseudovirions contain almost all of the different chloroplast DNA transcripts except for ribosomal RNA. Experiments are proposed to characterize pseudovirions and to determine their functional significance. Non-infectious TMV-like rods have been demonstrated to be present in chloroplasts. Tests will be made to determine whether these are pseudovirions. Is pseudovirion assembly like virion assembly in being initiated by a reaction between a capsid protein aggregate and a specific nucleotide sequence or structure? This will be determined by observing whether chloroplast transcripts become stable and specifically encapsidated when capsid protein is mixed with leaf RNA from uninfected plants under reconstituting conditions. If pseudovirions are formed "in vitro" then presumptive ei sites will be characterized by analyzing nucleotide sequences protected by minimal encapsidation of several chloroplast transcripts. A working hypothesis is that pseudovirions are assembled in the chloroplast from resident RNA species and capsid protein which has been transported into the chloroplast from its site of synthesis in the cytoplasm. This hypothesis will be tested by determining whether capsid protein is taken up by chloroplasts in vitro and, if so, whether by an energy dependent mechanism. A series of experiments are proposed to determine the possible significance, of pseudovirion formation for the biology of virus infection.